Ingenious cellulose-based materials were applied in smart piezoelectric and triboelectric nanogenerators for highly efficient energy harvesting.

Flexible and eco-friendly dielectric materials with high energy density and breakdown strength have promising applications in energy storage devices. Here, cellulose nanofibril/boron nitride nanosheet nanocomposite films were fabricated from 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs) and hexagonal boron nitride nanosheets (BNNSs) in aqueous dispersions. The obtained TOCN/BNNS nanocomposites were further cross-linked by immersion in aqueous CaCl₂ solution to improve their properties. The maximum discharged energy density of the finally prepared composite film was 3.9 J cm⁻³ with only 4 vol% of BNNSs at 380 MV m⁻¹, achieving greater enhancement than the neat TOCN film with a low energy density of 0.1 J cm⁻³ at 50 MV m⁻¹. It is much higher than the previously reported highest energy density of biopolymer-based composites (0.81 J cm⁻³) and also higher than that of the commercially available best dielectric polymer BOPP (2 J cm⁻³). Moreover, the nanocomposite films exhibited excellent mechanical flexibility and significantly improved thermal stability (100 °C increase in thermal decomposition temperature).

Although the elastocaloric effect was found in natural rubber as early as 160 years ago, commercial elastocaloric refrigeration based on polymer elastomers has stagnated owing to their deficient elastocaloric effects and large extension ratios. Herein, we demonstrate that polymer elastomers with uniform molecular chain-lengths exhibit enormous elastocaloric effects through reversible conformational changes. An adiabatic temperature change of −15.3 K and an isothermal entropy change of 145 J kg⁻¹ K⁻¹, obtained from poly(styrene-b-ethylene-co-butylene-b-styrene) near room temperature, exceed those of previously reported elastocaloric polymers. A rotary-motion cooling device is tailored to high-strains characteristics of rubbers, which effectively discharges the cooling energy of polymer elastomers. Our work provides a strategy for the enhancement of elastocaloric effects and could promote the commercialization of solid-state cooling devices based on polymer elastomers.

With increasing interest and demand for wearable sensor devices, fabricating a flexible hydrogel film sensor combining high strength and high sensitivity for monitoring human physiological signals is necessary but remains challenging. Recently, we have successfully prepared flexible piezoresistive electronic skin (E-skin) using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) and sulfonated multi-walled carbon nanotubes (SCNTs). The flexible sensor exhibited an extremely high sensitivity of about 4.4 kPa⁻¹, an ultrafast response time below 10 ms, an ultralow detection limit of 0.5 Pa, good stability (>11 000 cycles) and a mechanical strength of up to 184 MPa. At the same time, the pressure sensor had the advantages of a simple preparation process, low time consumption, low cost and good biocompatibility. With the popularity of smart terminal devices, the flexible sensor has great application prospects in the fields of future wearable electronic devices, health monitoring, motion monitoring, intelligent prosthetics, and artificial intelligence.